1. Field of the Invention
The present invention relates to a display device which includes display pixels using luminescent elements of types different in luminous characteristics, particularly to a display device in which organic electro luminescence (EL) elements for emitting light in red, green and blue are used as the luminescent elements.
2. Description of the Related Art
In recent years, organic EL display devices have been regarded as promising monitor displays for portable information terminals since the devices have such characteristics as lightness, thinness, and high luminance. A typical organic EL display device includes a plurality of display pixels arrayed in a matrix form to display an image. In this organic EL display device, a plurality of scanning lines are disposed along rows of the display pixels, a plurality of signal lines are disposed along columns of the display pixels, and a plurality of pixel switches are disposed near intersections of the scanning and signal lines. Each display pixel includes an organic EL element, a driving element connected in series with the organic EL element between a pair of power terminals, and a capacitance element for storing the gate voltage of the driving element. Each pixel switch is turned on in response to a scanning signal from the corresponding scanning line to write or supply an analog video signal from the corresponding signal line to the gate of the driving element. The driving element supplies to the organic EL element a drive current corresponding to the analog video signal.
The organic EL element is of a structure having a luminescent layer formed of a thin film having a red, green, or blue luminescent material such as an organic compound and held between a cathode and anode so that electrons and holes are supplied and recombined in the luminescent layer to produce excitons. The organic EL element outputs light radiated upon deactivation of the excitons. The anode is a transparent electrode formed of ITO or the like, and the cathode is a reflective electrode formed of a metal such as aluminum. With this structure, the organic EL element can produce a luminance of about 100 to 100000 cd/m2 with an applied voltage of just 10 V or less.
In the case where the organic EL display device uses pluralities of display pixels comprising the organic EL elements for emitting light of different colors, for example, red (R), green (G), and blue (B), such luminous characteristics as the luminous efficiency and current-luminance characteristic generally differ between the display pixels for each color. Therefore, if a plurality of display pixels is uniformly driven according to gradation data, the white balance and gradation are distorted.
When the gamma correction is used to solve this problem, the scale of the drive circuitry for the display pixels increases, and it easily becomes difficult to incorporate the circuitry into a portable information terminal.